Of the many known types of pressure sensors, silicon sensors have had great market success, thanks especially to their reliability, low cost and compact dimensions.
The methods of production and operation of silicon sensors are known, and for this reason will not be described in detail herein, while only some characteristics important to the understanding of the invention will be described in greater detail.
Depicted in FIG. 1 is a silicon pressure sensor (10), constructed according to the known art: a silicon transducer or die (11) comprises a thin and flexible membrane (14), made by removing part of the silicon, for instance by chemical etching or by mechanical detachment; made using known techniques on an outer face (24) of the die, are piezoresistors (21) that are connected together so as to form a Wheatstone bridge.
When pressure is applied to the transducer (11), this causes the membrane (14) to flex, as a result of which the resistance of the piezoresistor (21) changes; in this way, when a voltage is applied to the circuit, a change in the current may be read when pressure is applied to the sensor.
The silicon transducer or die (11) is then glued, using for instance a silicon- based adhesive, to a plate (12), made of a material, pyrex, silicon or glass for example, that has a thermal expansion coefficient close to that of the transducer (11). When the sensor (10) is subjected to a change in temperature, the plate (12) undergoes an expansion similar to that of the die (11), thus avoiding stresses being brought about on the transducer (11) by a difference in expansion between the transducer (11) and the plate (12), which would cause read errors.
In turn the plate (12) is joined, using known techniques, to a support (13), which may be, for instance, of ceramic material or consist of a printed circuit.
Built on the support (13) is an electronic circuit, which is connected by way of bonding wires (22) to the piezoresistors (21) and which comprises components for amplifying and correcting the signal output by the sensor (10); the signal is in turn sent for reading to a board external to the sensor (10), not shown in any of the figures.
In many of the applications of pressure sensors, they have to be used in a hostile environment, which may for example consist of a fluid containing corrosive substances, or which is at high pressure or temperature.
In order to protect these sensors a resin shell is widely used (25), made using techniques known in the assembly of electronic components.
To allow the fluid to come into contact with the transducer (11), apertures (15) and (16) are made in the plate (12) and on the support (13), whilst an aperture (27) is made in the shell (25) that puts the sensor into communication with the outside, thus permitting the difference in pressure with the environment to be measured.
The piezoresistors (21) and the bonding wires (22) that are on the outer face (24) of the transducer (11) could be damaged by the action of an aggressive fluid; on the contrary, on an inner face (23) of the transducer (11) there are no delicate components and therefore there are no particular problems if the inner face (23) is in contact with the fluid.
For this reason, sensors have been produced for hostile environments, that are operated in such a way that the transducer (11) has its inner face (23) in contact with the aggressive fluid. In addition, the surface of the inner face (23) may be covered by a protective layer, made for instance of a layer made of alloys of chromium, tantalum, silicon carbide or others.
In these sensors, the outer face (24) of the die (11), which comprises delicate components such as the piezoresistors (21) and the bonding wires (22), is not in contact with the aggressive fluid.
In addition the transducer (11), the piezoresistors (21), the bonding wires (22), and the support (13), upon which the tracks of the electronic circuit are made, may be protected even further by covering them with a gel or a protective resin (26).
Should the fluid the pressure of which has to be measured, act on the opposite face of the sensor, i.e. the outer face (24), therefore passing through the aperture (27) of the shell (25), the gel (26) performs the function of protecting the outer face (24) of the transducer (11), with the piezoresistors (21) and the bonding wires (22), from the action of the same fluid.
In the latter case however, the gel (26) does not guarantee the surface of the outer face (24) of the die (11) a sure protection, as it may be damaged by the action of the aggressive elements present in the fluid.
The silicon sensors for hostile environments made according to the known art have a number of drawbacks: the protections needed for applications in hostile environments complicate construction of the sensor, requiring assembly of a large number of parts and sometimes the production of costly fixtures.
Another problem, where there are high pressures and an aggressive environment, may be caused by the join between the transducer (11) and the plate (12), which may not be resistant enough to support high stresses.
Moreover the application in hostile environments may give rise to applied tensions on the transducer (11), caused by sudden jumps in temperature or by transitory extra-tensions induced on the support (13) by the fluid itself, with resultant read errors.
In the sensor (10) made according to the known art, the plate (12) and the transducer (11) are glued above the support (13), and are therefore protruding with respect to the surface of the support
This assembly leaves the transducer (11) more exposed to the action of external transitory forces, caused by the fluid itself, which can cause momentary incorrect pressure value readings or even damage the transducer.